Couple of noob questions re gas boiler

mr chips

I'm setting up a small campsite in NI for next year and I'll be installing eleven showers in total - the only requirement is to produce hot water for the showers & for five or six sinks for hand-washing, i.e. no space heating. As it's not a replacement of a domestic boiler but rather a new commercial install, there are no specific grants for this available here, so I'll be paying the full cost of the system. There's no mains gas here so I'll be going with the big LPG cylinders, at least initially. I also plan to set it up that I can add solar water heating (i.e. thermal rather than PV) in my second year of trading - not expecting many visitors outside the March-September window, so longer daylight hours for pre-heating water would coincide with the period of highest demand.

To manage the water & heating costs, the plan is to install restricted flow shower heads & taps, with the sort of time-controlled switches you push to get e.g. 1 minute of water flow before it clicks off again (more like 10-15 seconds for the sinks). On that basis I've been advised that I need a 300 litre hot water generator and thus far, the best (purchase only) quote I have for that is £1375+VAT/£1650 Net, with a further £1600+VAT/£1920 Net for a 40KW LPG boiler (Worcester Bosch Gas Boiler c/w Horizontal flue kit). Installation costs not included.

Do those specs sound right? Is the Bosch gas boiler a good choice (e.g. how good are they at standing over their warranty), or if not what alternatives should I investigate instead? As per thread title, I'm not knowledgeable about this stuff so I don't know what other questions to ask - if it looks like I haven't considered something basic/important I'd be glad to hear it.

John.G

mr chips wrote: »

I'm setting up a small campsite in NI for next year and I'll be installing eleven showers in total - the only requirement is to produce hot water for the showers & for five or six sinks for hand-washing, i.e. no space heating. As it's not a replacement of a domestic boiler but rather a new commercial install, there are no specific grants for this available here, so I'll be paying the full cost of the system. There's no mains gas here so I'll be going with the big LPG cylinders, at least initially. I also plan to set it up that I can add solar water heating (i.e. thermal rather than PV) in my second year of trading - not expecting many visitors outside the March-September window, so longer daylight hours for pre-heating water would coincide with the period of highest demand.

To manage the water & heating costs, the plan is to install restricted flow shower heads & taps, with the sort of time-controlled switches you push to get e.g. 1 minute of water flow before it clicks off again (more like 10-15 seconds for the sinks). On that basis I've been advised that I need a 300 litre hot water generator and thus far, the best (purchase only) quote I have for that is £1375+VAT/£1650 Net, with a further £1600+VAT/£1920 Net for a 40KW LPG boiler (Worcester Bosch Gas Boiler c/w Horizontal flue kit). Installation costs not included.

Do those specs sound right? Is the Bosch gas boiler a good choice (e.g. how good are they at standing over their warranty), or if not what alternatives should I investigate instead? As per thread title, I'm not knowledgeable about this stuff so I don't know what other questions to ask - if it looks like I haven't considered something basic/important I'd be glad to hear it.

Based on the above it would seem that you are considering a shower restricted flow rate of 5 LPM, is this correct?, if so then 300 litres of stored water at 60C should provide adequate hot water for the 11 showers running simultaneously for 5 minutes each and still have a reserve for basins etc. Have you any spec on the hot water cylinder/generator as some of these are a "tank within a tank" design and give very fast recovery times, others have fast recovery time coils to achieve the same thing, the boiler would then be sized to this performance.

mr chips

Glad you asked - the restrictors I've looked at are 6 litres per minute rather than 5, but I could probably cut that back a little by having a timer pop off after e.g. 30-40 seconds instead of 60. I wouldn't want it to be any less than that as it'd a bit of a PITA for customers to have to keep pressing it.

Would the water need to be heated as high as 60C? I thought the recommended maximum temperature for showers was something like 45 - I don't have paperwork to hand right now, but I'm pretty sure that in a care situation, it's limited to 38C (this may be relevant for me as I'm planning to install a fully accessible bathroom with hoist etc once I've been open a year).

I've had a few recommendations for this hot water cylinder arrangement or this one from the same outfit, but the cost for either would be a bit higher at £1600+VAT on top of the same amount for the boiler, so £3840 Net. The extra £270 probably isn't significant in the grand scheme of things and they provide a 25 year warranty, but I haven't yet been given the warranty details for the cheaper hot water generator so don't know at this stage if I'm comparing like with like.

John.G

mr chips wrote: »

Glad you asked - the restrictors I've looked at are 6 litres per minute rather than 5, but I could probably cut that back a little by having a timer pop off after e.g. 30-40 seconds instead of 60. I wouldn't want it to be any less than that as it'd a bit of a PITA for customers to have to keep pressing it.

Would the water need to be heated as high as 60C? I thought the recommended maximum temperature for showers was something like 45 - I don't have paperwork to hand right now, but I'm pretty sure that in a care situation, it's limited to 38C (this may be relevant for me as I'm planning to install a fully accessible bathroom with hoist etc once I've been open a year).

I've had a few recommendations for this hot water cylinder arrangement or this one from the same outfit, but the cost for either would be a bit higher at £1600+VAT on top of the same amount for the boiler, so £3840 Net. The extra £270 probably isn't significant in the grand scheme of things and they provide a 25 year warranty, but I haven't yet been given the warranty details for the cheaper hot water generator so don't know at this stage if I'm comparing like with like.

Does the timer just create a "nuisance" effect so that people get fed up with pressing it and has/is it proven to be effective?.
Bearing in mind that the standard electric shower will at best give 4 LPM in the summer then 6 LPM might be a bit of "overkill"? but most of the minimum shower heads/restrictors seem to be sized for this.
Looking at the two generators both heat the domestic/shower water by passing it straight through a heat exchanger so in that case it doesn't need to be heated to 60C but to get rapid heat exchange the boiler water has to be heated to a very high temperature...I've seen 85C quoted for some of these high flow units but that's no problem for the boiler.
The first units specifications state "Heating load maximum" 23 KW and "Hot water Allowance" 11 KW, what does this mean?.
If you need to run 11 showers together at a flow rate of 6 LPM at 45C from mains at say 12C (summer) the heat exchanger required would have to be 152 KW, if the heat exchanger is 23 KW then ~ 2 showers can be run together so it looks as if you need stored domestic hot water for your requirements?.

mr chips

Re the timer - essentially yeah, nobody's going to stand under the spray for 15-20 minutes at a time if they're having to click it on again every 30-40 seconds. I'm not a huge fan of that sort of set-up myself but when I'm the one paying for the heat and the water I have to control it somehow, and it's preferable to a coin- or token-operated arrangement which would be time-consuming for me and/or increase the amount of petty cash on site. Also means that nobody will walk away from using a sink and leave the hot tap running.

I didn't know that about electric showers, always thought that they would use more water than that. Our own showers at home aren't electric, they run off the tank and the water pressure here is high enough that if you turn them up full blast they'd nearly strip the skin off you. In fact, to get a rough idea I've just checked how quickly the shower can fill a 3L container and it would easily put out 12 litres a minute at full blast. Probably worth investigating other restricted-flow heads to see if I can get something as low as 4 litres per minute, so long as that doesn't turn a shower spray into a miserable dribble altogether.

I'll check with the supplier about the power output of the heat exchanger. I did specify that I would need to run up to 11 showers simultaneously and looking back over notes, he advised that 300L would be all the size of tank I should need.

Plenty of food for thought - your input is much appreciated.

John.G

mr chips wrote: »

Re the timer - essentially yeah, nobody's going to stand under the spray for 15-20 minutes at a time if they're having to click it on again every 30-40 seconds. I'm not a huge fan of that sort of set-up myself but when I'm the one paying for the heat and the water I have to control it somehow, and it's preferable to a coin- or token-operated arrangement which would be time-consuming for me and/or increase the amount of petty cash on site. Also means that nobody will walk away from using a sink and leave the hot tap running.

I didn't know that about electric showers, always thought that they would use more water than that. Our own showers at home aren't electric, they run off the tank and the water pressure here is high enough that if you turn them up full blast they'd nearly strip the skin off you. In fact, to get a rough idea I've just checked how quickly the shower can fill a 3L container and it would easily put out 12 litres a minute at full blast. Probably worth investigating other restricted-flow heads to see if I can get something as low as 4 litres per minute, so long as that doesn't turn a shower spray into a miserable dribble altogether.

I'll check with the supplier about the power output of the heat exchanger. I did specify that I would need to run up to 11 showers simultaneously and looking back over notes, he advised that 300L would be all the size of tank I should need.

Plenty of food for thought - your input is much appreciated.

Yes you are quite right re the 12 LPM, the electric showers that I'm referring to have the (electric) heating element inside the shower unit itself and also may have a integral pump in that unit and are fed from a COLD header tank in the attic or, like mine, are fed directly from the mains. The heating elements in these range from 7.5 KW to 10.5 KW with 9 KW being the norm, the 9 KW element is only powerful enough to heat 3 to 4.3 LPM from cold to normal showering temperatures depending on the cold water temperature which can range from 5C in winter to +15C in summer. A very high percentage of homes would have these showers so quite a few of the campers would be quite used to these flow rates and it is a campsite not a 5 Star Hotel.
You will have to be absolutely certain that you are getting the type of cylinder that you require, I would be 99% certain that neither of those heat exchangers, (if used for directly heating the domestic hot water)
will have remotely anything like the 150 KW power required, however it might be possible to make one to your requirements of 150 KW output but again be absolutely sure of this in writing.

Now if you have a 300 Litre cylinder full of hot water at 60C then that will supply over 430 litres of water at 45C for whatever purpose is required, it will feed all 11 showers simultaneously for 5 minutes and still have over 70 Litres left over (100 Litres at 45C) for other purposes.
This cylinder can be heated with a 3 KW or 6 KW or higher power electric heating element but would take over 5.5 hrs to heat/reheat using the 3 KW element or 2.8 hrs with a 2 X 3 KW elements or < 2 hrs if you use 3 X 3 KW elements.
IF you go for a hot water heating coil (primary) then you can put in a rapid recovery time coil which, depending on coil type, surface area and circulation rate can have heating power of anything up to 60 KW, IMO. If you have a 40 KW boiler running at full output and a suitably sized coil then you can heat/reheat 300 litres to 60C in < 25 minutes.
You will have to heat the cylinder to 60C, irrespective of usage temperature, for legionella protection.

You may be pretty set on the LPG route with its potentially very fast cylinder recovery times but LPG is the second most expensive form of energy after full cost electrical power ( in "Ireland" at any rate) but in NI the LPG may be cheaper or you may be able to get cheaper night/day time electrical rates, it may be worth just having a quick look at that option. With either LPG or electric you can still install solar at a later date if the cylinder has the coil pre installed.

Borzoi

You should definitely look at an electric A2W heat pump for this application. Might cost a little more on the install but would definitely be cheaper to run and wouldn't require an LPG tank.

Because you're only running during summer months the water temps wouldn't be an issue. You'll need approx 45C for hot showers with an occasional hot boost to ensure legionella control

John.G

Borzoi wrote: »

You should definitely look at an electric A2W heat pump for this application. Might cost a little more on the install but would definitely be cheaper to run and wouldn't require an LPG tank.

Because you're only running during summer months the water temps wouldn't be an issue. You'll need approx 45C for hot showers with an occasional hot boost to ensure legionella control

I suppose one of the problems with the domestic type ones like the Ariston Nuos (there is a thread on here re them) is their capacity, I think the Nuos has a 0.75 KW compressor so even with a COP of 4 it will still only produce ~ 3 KW of heating which is probably far too low for the OP. There are probably far bigger ones?.

mr chips

Unfortunately, there are issues which mitigate against the heat-pump option. Firstly, the amount of hot water storage I would have to install would be significantly greater than with with the gas option. Secondly, while I would much rather have an air-source or ground-source heat pump, the upfront cost is too high for me to manage it on my own and I'll be applying for a grant for 50% of certain capital costs. Believe it or not, one of their stipulations is that they will not fund any renewable energy projects! I can only assume that this is an after-effect of the Renewable Heat Initiative scandal here, at least while the public inquiry continues.

That being the case, to get the project off the ground I will probably have to install the gas boiler, run it for e.g. three years and then consider selling the lot in order to part-fund the cost of an air-source heat pump. An oil-fired boiler wouldn't be desirable because of the acrid smell of the fumes wafting around the campsite, unless I spend out on a tall vertical flue which would be visually unappealing - but more fundamentally, I have to minimise my upfront costs for now and gas burns a lot cleaner.

John.G

mr chips wrote: »

Unfortunately, there are issues which mitigate against the heat-pump option. Firstly, the amount of hot water storage I would have to install would be significantly greater than with with the gas option. Secondly, while I would much rather have an air-source or ground-source heat pump, the upfront cost is too high for me to manage it on my own and I'll be applying for a grant for 50% of certain capital costs. Believe it or not, one of their stipulations is that they will not fund any renewable energy projects! I can only assume that this is an after-effect of the Renewable Heat Initiative scandal here, at least while the public inquiry continues.

That being the case, to get the project off the ground I will probably have to install the gas boiler, run it for e.g. three years and then consider selling the lot in order to part-fund the cost of an air-source heat pump. An oil-fired boiler wouldn't be desirable because of the acrid smell of the fumes wafting around the campsite, unless I spend out on a tall vertical flue which would be visually unappealing - but more fundamentally, I have to minimise my upfront costs for now and gas burns a lot cleaner.

You have done your research well, what have you calculated your daily hot water demand at? and what capacity gas cylinders are you using and cost/kwh?

According to the July SEAI link, the cost of using LPG cylinders (doesn't specify size) is €0.1398/Kwh.

mr chips

Basing these figures on maximum demand - correct me if I'm wrong!!

11 showers x 6 litres per minute x 7 minutes per shower (erring a little on the side of caution there) = 462 litres for all 11 showers.
6 hot water basins x 1 litre per minute x 7 minutes (i.e. if all showers are in use simultaneously) = 42 litres. So one full "cycle" would use 500 litres, assuming both accessible bathrooms are also in use.

Maximum campsite capacity is 100 people, so e.g. on bank holiday weekends etc the above essentially has to be capable of happening 9 times if every single person takes a shower and uses hot water to wash hands in the morning (I know that's not hugely likely, but I can well imagine that it would reach at least 80% of demand). So just for hot water, the maximum need could reach 4500 litres in a day ...

Last time I checked, the domestic price for a 47kg cylinder of propane was £93. That's a few months ago now though, plus the commercial rate would be a bit lower. But I don't have that figure to hand in any case, so for ease of calculation I'll just say £90 and assume a 40kW boiler is running for an hour a day instead of the 63 minutes which would be the daily maximum as calculated above.

So running at peak demand every day, a 47kg cylinder would last me 16-18 days in peak season, but obviously I'm only expecting to operate at maximum capacity 2-3 days a week - the more likely figure for weekly demand through the high season would average around 60% capacity, with a few peaks. So I should get around three & a half weeks from a 47kg cylinder, or roughly £670-£730 over the 26 weeks from April-September. Occupancy rates would plummet in the October-March period, so I'd probably get away with using one cylinder in that entire time.

John.G

mr chips wrote: »

Basing these figures on maximum demand - correct me if I'm wrong!!

11 showers x 6 litres per minute x 7 minutes per shower (erring a little on the side of caution there) = 462 litres for all 11 showers.
6 hot water basins x 1 litre per minute x 7 minutes (i.e. if all showers are in use simultaneously) = 42 litres. So one full "cycle" would use 500 litres, assuming both accessible bathrooms are also in use.

Maximum campsite capacity is 100 people, so e.g. on bank holiday weekends etc the above essentially has to be capable of happening 9 times if every single person takes a shower and uses hot water to wash hands in the morning (I know that's not hugely likely, but I can well imagine that it would reach at least 80% of demand). So just for hot water, the maximum need could reach 4500 litres in a day ...

Last time I checked, the domestic price for a 47kg cylinder of propane was £93. That's a few months ago now though, plus the commercial rate would be a bit lower. But I don't have that figure to hand in any case, so for ease of calculation I'll just say £90 and assume a 40kW boiler is running for an hour a day instead of the 63 minutes which would be the daily maximum as calculated above.

So running at peak demand every day, a 47kg cylinder would last me 16-18 days in peak season, but obviously I'm only expecting to operate at maximum capacity 2-3 days a week - the more likely figure for weekly demand through the high season would average around 60% capacity, with a few peaks. So I should get around three & a half weeks from a 47kg cylinder, or roughly £670-£730 over the 26 weeks from April-September. Occupancy rates would plummet in the October-March period, so I'd probably get away with using one cylinder in that entire time.

Re the max demand of 4500 litres of hot water daily, my calcs show that the energy required to heat that volume of water from 12C to 45C would be 173 KWH requiring the boiler to run for 4.3 hrs daily, 40 Kwh would only heat 1042 Litres?

mr chips

Really? Would you mind forwarding that to me, as that would obviously make a big difference to both running costs and upfront costs in terms of the power output of the boiler. Now is the time for me to iron this stuff out ...

John.G

mr chips wrote: »

Really? Would you mind forwarding that to me, as that would obviously make a big difference to both running costs and upfront costs in terms of the power output of the boiler. Now is the time for me to iron this stuff out ...

OK, here is just one of a few useful formulae that I have used for the past 40 years or so:

Litres X DeltaT / 860 = Kwh

from above, 4500*(45-12)/860 = 172.7 Kwh.

A 40 Kw output boiler would run for, 172.7/40, or 4.3 hrs/day.

the energy usage based on a boiler @ 85% efficiency would then be, 172.7/0.85, or 203 Kwh/day.
The 47 Kg cylinder @ 651 Kwh would last, based on the above, 651/203, or 3.2 days.

mr chips

Sorry, what's DeltaT?

I was working on the following -
Cylinder Kg x 1.957 = Litres
x7.11 = Kw energy of cylinder
divide by appliance input
= Total hours use of cylinder

47 x 1.957 = 91.979 litres
x 7.11 = 653.97 Kw
divide by 40 (Kw boiler) = 16.34 hours, or 16 and a bit days at an hour a day.

So have I gone wrong on the last bit?

John.G

mr chips wrote: »

Sorry, what's DeltaT?

I was working on the following -
Cylinder Kg x 1.957 = Litres
x7.11 = Kw energy of cylinder
divide by appliance input
= Total hours use of cylinder

47 x 1.957 = 91.979 litres
x 7.11 = 653.97 Kw
divide by 40 (Kw boiler) = 16.34 hours, or 16 and a bit days at an hour a day.

So have I gone wrong on the last bit?

The Delta T is the difference in temperature between the hot water (45C) and the incoming cold water (12C) average temperature that I have assumed for spring/summer.

Your actual calculation is OK as such if you only fired the boiler for 1 hour per day, now a 40 kw boiler at 85% efficiency will consume 40/0.85 or 47.1 Kwh so the 654 KWH gas cylinder would last for 654/47.1 or 13.9 days...a bit less than your calculation because you didn't take the boiler efficiency in to account BUT that has nothing to do with the actual hot water demand, as I said above if you fired the boiler for exactly one hour then you will raise 1042 litres of water from 12C to 45C, my formula states Litres X deltaT /860 = Kwh so it follows
from that...Litres = Kwhx860/Deltat, = 40*860/(45-12), or 1042.4 Litres.
BUT your requirement is four times this amount so that calc is meaningless really.

mr chips

Guess it was naive of me to think that a 40kW boiler would actually output 40kW of heat ...:pac: Right, so it looks like I'll need a bigger boiler (what's your view on pellet boilers, by the way?)

I had been thinking in terms of adding solar pre-heating for the water in my second year of trading, but maybe it's worth bringing that forward so as to have it in place from the very outset instead, even if it only/mainly serves to raise the starting temperature so that the boiler has less work to do.

I'd be capable of doing most of the setting up for something low-tech and low-cost with some radiators painted black and mounted under glass/clear polycarbonate on the roof of the shower block - I could easily add 5 square metres' worth (possibly 10, now that I think about it). However I suspect that this still wouldn't scale up to provide the sort of additional heat I really need - how warm could it get water in this climate during the summer? Plus I guess I'd need a bigger tank rather than the 300 litres I've been looking at, maybe 450 litres? It's late & I'm getting square-eyed ...

One other question - you talked about heating the water to 45C, and mentioned an "occasional boost" to 60C to eradicate legionella. How regularly would the 60C have to be reached?

John.G

mr chips wrote: »

Guess it was naive of me to think that a 40kW boiler would actually output 40kW of heat ...:pac: Right, so it looks like I'll need a bigger boiler (what's your view on pellet boilers, by the way?)

I had been thinking in terms of adding solar pre-heating for the water in my second year of trading, but maybe it's worth bringing that forward so as to have it in place from the very outset instead, even if it only/mainly serves to raise the starting temperature so that the boiler has less work to do.

I'd be capable of doing most of the setting up for something low-tech and low-cost with some radiators painted black and mounted under glass/clear polycarbonate on the roof of the shower block - I could easily add 5 square metres' worth (possibly 10, now that I think about it). However I suspect that this still wouldn't scale up to provide the sort of additional heat I really need - how warm could it get water in this climate during the summer? Plus I guess I'd need a bigger tank rather than the 300 litres I've been looking at, maybe 450 litres? It's late & I'm getting square-eyed ...

One other question - you talked about heating the water to 45C, and mentioned an "occasional boost" to 60C to eradicate legionella. How regularly would the 60C have to be reached?

Pellet Boilers seem to be a bit of a PITA with a lot of people switching back to Gas/Oil, one major problem seems to getting competent servicing if/when they break down, there are lots of posts on here about them.

Solar: Any homemade versions will not help you, the two commercial types are evacuated tubes and Flat Plate, evacuated tubes will give greater output in cloudy days but precautions have to be made to stop them overheating in days when there is not enough demand for their output, Flat Plate are less efficient but because of this you may not have to take precautions to avoid overheating.
There is really no pay back with either system or at best > 15/20 years IMO.
I would still suggest pricing a cylinder with the solar coil installed anyway and just maybe adding the solar system later.

Cylinder Sizing: I don,t have any experience in this area especially since you require enormous quantities of hot water, 4500 lires/day. When sizing for a normal house with say 4 occupants and allowing 60 Litres/day/person which is 240 litres/day, then it would be fairly standard practice to install a 250/300 litre hot water cylinder but I think it would be rather daft to suggest that you install a 5000 Litre "cylinder"!!!.

I would take professional advice before going any further and why not contact/visit a few campsites and see what they are doing.
You might end up opting for a mix of electric (9KW) showers and the remainder coming from a 500 Litre hot water cylinder.
I would also go back to the cylinder supplier, telling him exactly what your heating requirements are and see what he comes up with and post back.

Edit: Re Solar, I have a very modest 2 Flat Plate "4M2" solar array and a 150 litre twin coil (oil/solar) hot water cylinder and I monitored its performance for a good few years, based on the average of a two year period, this is what you might expect to get from both a 3 panel 6M2 array and a 4 panel 8M2 array, you could probably add ~ 25% to these outputs if you install the equivalent evacuated tube array.

Month Expected 3 Panel Output Daily KWH & Hot Water @ 45C litres Expected 4 Panel Output Daily KWH & Hot Water @ 45C Litres
March 2.95 77 3.9 102
April 5.4 141 7.2 188
May 6.3 164 8.5 222
June 6.7 175 9.3 242
July 7.0 182 9.3 242
Aug 6.2 162 8.3 216
Sept 4.9 128 6.5 169

mr chips

Yeah, the need for storage at that scale is looking ludicrous. In terms of seeking professional advice, I had a long & at times frustrating day today trying to follow this stuff up whenever I had the chance, and I've started to feel like certain people are trying to just sell me their gear rather than provide me with something that meets my needs.

I've started investigating Rinnai water heaters today and they look more promising - 2 water heaters at 180kW providing instant heat with no need whatsoever to store hot water. Installation cost is higher at a ballpark of £4200, but if that's what it takes to get what I need then I'll have to take it on the chin. It's not actually that much higher than the cost of boiler plus cylinder that I was working on up to now. There would be no need for any solar pre-heating, either.

Commercial price for Calor Biogas is 8p per kW. Domestic customers are paying over the odds!

John.G

mr chips wrote: »

Yeah, the need for storage at that scale is looking ludicrous. In terms of seeking professional advice, I had a long & at times frustrating day today trying to follow this stuff up whenever I had the chance, and I've started to feel like certain people are trying to just sell me their gear rather than provide me with something that meets my needs.

I've started investigating Rinnai water heaters today and they look more promising - 2 water heaters at 180kW providing instant heat with no need whatsoever to store hot water. Installation cost is higher at a ballpark of £4200, but if that's what it takes to get what I need then I'll have to take it on the chin. It's not actually that much higher than the cost of boiler plus cylinder that I was working on up to now. There would be no need for any solar pre-heating, either.

Commercial price for Calor Biogas is 8p per kW. Domestic customers are paying over the odds!

That looks very promising and even if one breaks down you can still run 5 or 6 showers together.
It might also be worth getting the water tested for hardness as these instant heaters are very highly rated and any build up of lime scale will kill them quickly.

I should have included the calculation for the power required based on flowrate....you might find it handy again some time.
Power required in KW = LPM X 60 X Delta T / 860, in the above case, theoretically, for 11 showers running at 6 LPM and 45C from cold water at 12C...
KW = 11*6*60*(45-12)/860 = 152 KW. Practically speaking, you will have to run at a higher temperature and use shower mixers as some users might want lower showering temperatures or even cold showers, if you run at 55C then because of the mixing, each shower will now only require 4.6 litres of water at 55C (plus 1.4 litres at 12C) so the power required will be the same, 11*4.6*60*(55-12)/860 = 152 KW.

The standard efficiency unit at 82.96% will consume 152/0.8296, 183 Kwh and the high efficiency unit at 95.01% will consume 152/0.9501, 160 Kwh.

Do you intend to install a bulk LPG tank or will you use the 47 Kg cylinders?.

mr chips

Thanks for that. My head's been melted with this stuff for the past few days so this has been really helpful.
I think I'll have to go with a bulk tank. It does mean that I'll have to install a concrete base and metal fencing that would be a lot bigger than what an enclosure for e.g. four cylinders would demand, but on the plus side I wouldn't have to go changing cylinders every few weeks. So long as I can ensure access for refilling etc, I'm pretty sure I've got a good spot to locate it.
If you ever decide to go camping in this neck of the woods, you can pitch your tent here for free ... Least I could do!